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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 30 March 2011 by Chris Colose

In Part 1, we outlined some general characteristics of stellar evolution. Notably, as the star converts Hydrogen into Helium in the core, its luminosity gradually goes up in time, and will eventually leave the main-sequence phase. For our own sun, it will then spend a relatively short time as a red giant with a more quickly evolving spectrum, and eventually collapse into a fading white dwarf.

It was brought up in the comments, and worth noting that while the stellar luminosity grows in time, it is usually less recognized that the flux at very short wavelengths (in the extreme ultraviolet, ~ 0.01-0.12 μm) was likely several orders of magnitude higher in the early stages of Earth’s history, and at least 2-3 times the present value at 2.5 billion years ago (e.g., Ribas et al., 2005). The energetic tail of the solar flux is dominated by the emission from high temperature plasma in the chromospheres and corona, not blackbody radiation produced within the stellar interior. These wavelengths are unimportant for planetary energy balance, but have vital implications for atmospheric photochemistry, atmospheric escape rates in the early stages, biology, and the stability of prospective greenhouse gases that may have been present. For example, the original Sagan and Mullen (1972) suggestion for helping to offset a faint sun was that ammonia (which can be a greenhouse gas) concentrations were higher on primordial Earth, however Kuhn and Atreya (1979) later showed that ammonia was photo-chemically unstable at any appreciable concentration, and would become rapidly photolyzed into H2 and N2.

Because the irradiation from the parent star is, by far, the most important source of energy in planetary atmospheres, we will take up the task of interrogating the implications of solar changes for climate. In the rest of this post, we will tackle two questions: what sort of mechanism might exist to help offset a fainter sun and keep early Earth or Mars capable of supporting liquid water? Secondly, what will happen to Earth (or other bodies) in the future as the sun continues to evolve?

Searching for a Thermostat

Earth has been subject to many large climate changes in the past; however, even with a substantially brighter sun later in Earth’s history, it has generally always been conducive to life. With the exception of a few brief snowball events recorded in the geologic record (which we managed to get out of, despite being very hard to do so), Earth has always supported vast amounts of liquid water. Even with Neoproterozoic solar insolation the Earth should be fully glaciated at modern-day CO2 levels, and even colder with early Archaean insolation, yet Earth managed to avoid persistent global glaciation for billions of years.

These observations lead us to believe some sort of thermostat that contains a negative feedback may help to offset the early sun. A lower albedo is one candidate, but early Earth would have to be almost completely black just to get you back to modern conditions with the same greenhouse effect. There’s also no known mechanism to adjust the albedo in such a way to keep a stable climate over geologic time, and the surface ice-albedo and water vapor feedback would lead to early Earth easily prone to glaciation. Instead, a well-accepted candidate is a greenhouse stabilizer known as the silicate-weathering feedback. How does it work?

Our planet is continually re-supplied with CO2 to the atmosphere through volcanism, which could double CO2 over just several thousands of years if operating on its own. CO2 is also removed over long timescales by weathering reactions. Atmospheric CO2 reacts with water to form a weak carbonic acid, which can then dissolve silicate rocks. The byproducts are calcium and bicarbonate ions along with dissolved silica, which can be carried by rivers and streams into the ocean. In the ocean, organisms use these to make shells of calcium carbonate (CaCO3) or silica; these organisms eventually die and settle to the ocean floor. Due to plate tectonic processes, these materials are then processed into Earth’s interior and eventually emitted back into the air through eruptions to complete the cycle (shown below).

To make this into a thermostat, we can note that volcanoes do not really listen to the climate, but on the other hand weathering rates depend on temperature and precipitation (Walker et al., 1981), such that enhanced weathering can draw down CO2 in warm/wet climates. In colder climates, weathering would be reduced, allowing CO2 to build up. The thermostat is estimated to be strong enough to reach equilibrium within a few hundred thousand years, so that shorter-lived fluctuations such as those over glacial-interglacial cycles are easily sustained for some time. Zeebe and Caldeira (2008) showed that carbon fluxes into and out of the Earth’s atmosphere have mostly been in balance over the long-term mean during the last 650,000 years, giving additional credibility to the thermostat.

The silicate weathering thermostat also serves to extend the orbital range of habitability around our sun (Kasting et al., 1993), since the negative feedback allows for greater flexibility in the conditions where liquid water can persist. There are still many twists in the thermostat, and it is quite evident that solid Earth sources and sinks of CO2 are not, in general, balanced at any given time; during times of unusual plate tectonic activity or mountain-uplift, the carbon imbalance can be large. There is still considerable work needed to be done, as well as debate within the community concerning how other hypotheses such as feedbacks involving organic carbon burial, or the “uplift mountain hypothesis” of Raymo and others, fit into the geologic evolution of the Earth (e.g., Raymo et al., 1988; Raymo and Ruddiman, 1992; Edmond and Huh, 2003; see link for one brief summary and above references). However, the silicate weathering thermostat helps put into perspective that CO2 plays a fundamental role in the evolution of Earth’s climate.

Increased weatherability also plays a role in understanding the Ordovician climate, a past period that skeptics have abused as evidence that CO2 has little effect on climate. Young et al (2009) proposed that there was enhanced basaltic weathering beginning in the mid-Ordovician that continued through the end of the Ordovician, also a time period with increased volcanism in North America. By the Upper Ordovician, volcanism returned to normal conditions but weathering remained high, such that CO2 concentrations were drawn down and the familiar Hirnantian glaciation was initiated.

The post-snowball Earth setting provides another opportunity to test how robust the thermostat mechanism is. In the geologic record we see carbonate deposits (cap carbonates) that indicate extreme carbonate supersaturation in the ocean and rapid deposition. This is a mainindicator for Neoproterozoic glaciations and deglaciations involving transitions between completely different states. Cap carbonates are continuous layers of limestone and/or dolostone that overlie Neoproterozoic glacial deposits. Transfer of atmospheric CO2 to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters, producing cap carbonate rocks.

A Future Outlook

Evidently, the silicate-weathering thermostat does not operate on neighboring planets, either because they lost water (e.g., Venus) or were small enough to lose substantial tectonic activity and forbid release of CO2 back into the air (e.g. Mars). These observations lead us to believe that a thermostat can only operate within a reasonable range of conditions, and these should be understood to explore habitability limits and Earth's future.

As the sun brightens in time, Earth will eventually get quite hot and allow for significant loss of water to space. Kasting (1988), following on previous work (Ingersoll, 1969), determined that Earth will get to a point in which even the stratosphere is rather wet and substantial amount of water can photodissociate and be lost. Substantial water loss occurs at ~10% increase in solar luminosity above today's value. At 140% of today’s solar luminosity, a full-fledged runaway greenhouse is possible, in which liquid water is incompatible on Earth’s surface. This occurs because the longwave emission of planetary atmospheres that contain a condensable absorbing gas in the infrared, which is in equilibrium with its liquid phase at the surface, can exhibit an upper bound. Pushing the absorbed shortwave radiation over this threshold makes a new radiative equilibrium impossible, at least until the oceans are depleted or the planet gets hot enough to start losing a lot of radiation in visible wavelengths.

On Venus, following the runaway greenhouse it is likely that CO2 was free to accumulate in the atmohere once the weathering was inhibited by water loss, resulting in the ~90 bar CO2 atmosphere we see today.

As the sun evolves, all solar system objects should get hotter, and the potential for habitability may also be pushed outwards. In the red giant phase, the surface of the sun should actually cool (diminishing the UV flux as well), but the luminosity will increase as its radius does. Interestingly, Lorenz et al (1997) found a brief window of a few hundred million years, about 6 billion years from now, in which Saturn’s moon Titan will be compatible with liquid water-ammonia at the surface. By then, Earth will be incinerated.

BP: technically possible, but would it ever happen? Given the warnings given by scientists about problems a century hence due to global warming, and the marked lack of action by world governments, I hold grave doubts that they'd work to prevent a problem a few hundred million centuries down the track. :-P

I recall a Larry Niven novel (Ringworld, I think) where a species of aliens was moving their entire solar system out of the galaxy due to some impending catastrophic wave of supernovae... that's probably beyond us at this point in time, but a billion years from now, if we're still around?

The extremely high Deuterium-Hydrogen ratio on Venus suggests that there was a lot more water in its early stages than is now (and it still has some trace amounts of water vapor in the atmosphere) although there's good indication it was never within the last billion years or so, so it must have been earlier. The timeframe over which water was lost is rather unknown, and if Venus went through a "moist greenhouse" phase could remain habitable until the timescale of hydrogen loss (in this case, an ocean can still be at the surface).

Thanks Chris, that's kind of what I thought. So it's entirely possible that Venus might have had liquid water around for a few hundred million or even a billion years or more, before it all boiled off. I wonder how close it was - whether it was only a few watts/m2 over the limit, or if it was more than that.

I seem to recall Hansen did a lot of his earlier work on greenhouse effect on Venus, I'll have to do a bit of reading of his early work!

Typical inner limits push the orbital distance for substantial water loss at ~0.95 AU (for our own sun) and down to about 0.84 AU for a true runaway greenhouse (e.g., Selsis et al., 2007). Jim Kasting has some of the best work on this; Bullock and Grinspoon (2001) paper is another must read if you're interested in the evolution of Venus.

The problem is that so much of these estimates are based on clear-sky physics, or clouds only very crudely modeled, because we don't really know how well they help to offset the increased luminosity and make a runaway harder to achieve. They could also make a runaway easier, but that's probably not the case because the gaseous form of water swamps anything else in the atmosphere in the near-runaway regime.

If we accept that Venus was liquid water free a billion years ago, then we can at least say that the inner orbital limit is its current distance of 0.72 AU multiplied by (1/0.92)^0.5, or ~0.75 AU (based on the Gough Equation in my first post, the solar luminosity was 8% less, and the flux goes as the inverse square of the distance). If it was water free 2 billion years ago, then we can narrow that to about 0.85 AU. Unfortunately we don't know the timescales of water loss or how large its initial water inventory was with any confidence.

Interesting concept and totally possible with only a moderate development of our space travel capability.

The key sticking point is not technical but psychological. At present our human psychological development has not progressed to the point where we could consider such a long term project. Millions of years from now I surely hope we have developed to that point.

Perhaps our response to AGW is one of the first tests of that, the first hurdle. Can we bring enough emotional maturity and dispassion to our thinking on this, as a species, to solve the problem?

"Not necessarily. It depends on how hard posterity is willing to work."

@3 Bern:

"Given the warnings given by scientists about problems a century hence due to global warming, and the marked lack of action by world governments, I hold grave doubts that they'd work to prevent a problem a few hundred million centuries down the track. :-P

No, the "world governments" of today aren't going to be in a position to do anything a thousand years from now. More importantly, if Humanity survives, it will be a different species alltogether.

A species lacking in the genetically predisposed behaviors of Dominance/Submission; the hierarchical societies that it creates; and psychopathology; is the only type of Humanity that can survive.

So it's either a radically different Humanity (behaviorally speaking) that will inherit the future or none at all.

A lot of things will be possible for that kind of Humanity, including relocation of Venus/Mercury and Earth/Moon. A terraformed Mercury would make a nice satellite for a terraformed Venus.

Could you elaborate on the "status" of this thermostat hypothesis, in comparison with competing hypotheses (of which you metnion the organic CO2 burial and uplifting)?

I'm thinking of a "consilience of evidence" style argument: How strongly are the different hypotheses supported by the combined evidence? and in relation to that, how would you characterize the disagreement about these hypotheses amongst experts?

I'm trying to get a feel of how strong the evidence and, as a consequence, the consensus is about this topic.

This review article back in 2000 is a good start
http://www.essc.psu.edu/~brantley/publications/kump.pdf

Since then, there's been a number of field studies (e.g., Dessert et al., 2001; Oliva et al., 2003; Gislason et al., 2008; Turner et al., 2010) that show weathering rates can go up with temperature and runoff, however there's also dependence on topographical factors, rock exposure and soil overlying that rock, etc which make for complications. There's also been debate when it comes to interpreting Strontium isotopes or other ways of diagnosing weathering rates in the past, but in general I don't sense much disagreement that this serves as an important negative feedback over sufficiently long timescales.

I mentioned the Zeebe and Caldeira paper (see also David Archer's summary of that paper) simply because it was useful in testing an elusive (and largely theoretical) mechanism over the time-frame where ice cores exist.

Another important point in the review article (also discussed in Hansen's Target CO2 paper) is that the CO2 inventory in the atmosphere is very small when compared to the Earth sources and sinks, so increased removal of CO2 with uncompensated output by volcanism after a Himalayan uplift would draw levels down to zero in a million years or so. This means a counterbalancing process is necessary, largely provided by a deceleration globally of weathering in regions un-impacted by the mountains due to temperature, which would help control the drawdown of CO2 in the Cenozoic.

Regarding the cap carbonates, there's an article by Greg Retallack (Neoproterozoic loess and limits to snowball Earth), in which he argues that at least the Aussie Nuccaleena Formation is actually a subaerial loess deposit, and therefor nothing to do with the termination of Snowball Earth, and by extension don't support CO2 as a major player in terminating these ice ages.

I asked my old Lecturers at Adelaide Uni who worked on these formations for comment, and they were less than complimentary about the science - apparently it had been previously roundly rejected by peer review.

So, just in case someone brings this article up to 'debunk' CO2's role, the article is apparently not very solid.

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